<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        #box1{
            position: relative;
            width: 800px;
            height: 100px;
            background-color: aqua;

        }
        #box2{
            position: absolute;
            background-color: #3a4;
            top: 150px;
            bottom: 0;
            left: 0;
            right: 0;
            margin:0;
            padding: 0;
        }
    </style>
</head>
<body>
    <div id="box1">
        <div id="box2"></div>
    </div>
</body>
</html>